Electrode paste for solar cell, solar cell using the paste, and fabrication method of the solar cell

ABSTRACT

An electrode paste for a solar cell, a solar cell electrode using the paste, a solar cell having such an electrode, and a fabrication method of the solar cell are described. The paste for a solar cell electrode comprises a first component that includes silver (Ag) or a metal alloy containing the silver (Ag); a second component that includes zinc (Zn), and at least one selected from a group consisting of silicon (Si), aluminum (Al), copper (Cu), manganese (Mn), bismuth (Bi), phosphorous (P), boron (B), barium (Ba), and palladium (Pd); a leaded or lead-free glass frit; and a resin binder that is dispersed in an organic medium.

This application claims the benefit of Korean Patent Application No.10-2009-0061251 filed on Jul. 6, 2009, which is hereby incorporated byreference for all purposes as if fully set forth herein.

FIELD OF THE INVENTION

The present invention relates to an electrode paste for a solar cell, asolar cell electrode using the electrode paste, a solar cell having anelectrode along with the paste, and a method of fabricating the solarcell, and more specifically, to an electrode paste which improves theefficiency of a solar cell by controlling the depth of the emitter in asilicon solar cell.

BACKGROUND OF THE INVENTION

Because of the rapid rise in oil prices, environmental concerns, theexhaustion of fossil energy, waste treatment associated with nuclearpower generation and the like, interest in renewable energy hasdramatically increased. In particular, there has been increased interestin the research and development of solar cells, which are apollution-free energy source.

Solar cells are been advantageous because they are a pollution-free,sustainable resource with a semi-permanent life span. It is believedsolar cells can ultimately solve our energy problems.

Solar cells are categorized into various types: silicon solar cells,thin film solar cells, dye sensitized solar cells, organic polymer solarcells and the like, according to its constituent material. Inparticular, crystalline silicon solar cells make up the majority ofsolar cells throughout the world. Crystalline silicon solar cells have ahigher efficiency compared to other solar cells and techniques to lowerthe unit cost of manufacturing is continuously being developed.

In order to further improve the efficiency of crystalline silicon solarcells, there are ongoing studies to increase the short circuit current(Isc), the open circuit voltage (Voc), and the fill factor (FF). Thepresent invention is also concerned with improving efficiency and morespecifically to an electrode paste for use with high efficiency solarcells.

there are also ongoing studies relating to ultra-thin solar cells, whichare easy to handle and are very versatile. Thus, there is a need toimprove the manufacturing process for forming ultra-thin emitters onultra-thin solar cell wafers.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, the variousobjectives and advantages are realized by a solar cell. The cellcomprises, among other things, a semiconductor substrate comprising afirst conductive type material and a semiconductor layer comprising asecond conductive type material formed on a front surface of thesemiconductor substrate, where the first conductive type material isdifferent than the second conductive type material. The solar cell alsocomprises an antireflective layer formed on the semiconductor layer, afront electrode passing through the antireflective layer and contactingthe semiconductor layer; and a rear electrode formed on a rear surfaceof the semiconductor substrate. The depth of the semiconductor layer is100 nm to 500 nm and the front electrode comprises a first componentincluding silver (Ag) and a second component including zinc (Zn).

In accordance with a second aspect of the present invention, the variousobjectives and advantages are realized by a solar cell fabricationmethod. The method involves forming a semiconductor layer having a depthof 100 nm to 500 nm by doping a front surface of a semiconductorsubstrate using a second conductive type material impurity, where thesemiconductor substrate comprises a first conductive type material andthe first conductive type material is different from the secondconductive type material. Next, the method involves forming anantireflective layer on the semiconductor layer, printing a frontelectrode paste on the antireflective layer; printing a rear electrodepaste on a rear surface of the semiconductor substrate and heat-treatingthe front electrode paste thereby causing a resulting front electrode topenetrate through the antireflective layer and make contact with thesemiconductor layer, where the composition of the front electrode pastelimits the penetration of the front electrode to the semiconductorlayer. The rear electrode paste is then heat-treated thereby resultingin the formation of a rear electrode.

The foregoing provides exemplary embodiments of the invention. Thoseskilled in the art will appreciate that the exemplary embodiments areillustrative only and not intended to be in any way limiting. Thevarious aspects, features, and advantages of the present disclosure, asdefined solely by the claims, will become apparent in the detaileddescription set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

These aforementioned and other objects, features, aspects and advantageswill be fully described in the following detailed description ofexemplary embodiments, taken in conjunction with the accompanyingdrawings. In the drawings:

FIGS. 1 to 5 are cross-sectional views sequentially showing afabrication method of a solar cell according to one exemplaryembodiment;

FIG. 6 is a cross-sectional view showing a solar cell manufactured bythe fabrication method according to the one exemplary embodiment;

FIG. 7 is a flow chart of the fabrication method according to the oneexemplary embodiment; and

FIG. 8 is a graph comparing the doping concentrations of a semiconductorimpurity according to the sputter depths of emitter layers in a High Rscell and an existing solar cell.

DETAILED DESCRIPTION

In the following disclosure, an electrode paste for a solar cell isdescribed. According to one exemplary embodiment the electrode pastecomprises: a first component including silver (Ag) or a metal alloycontaining the silver, a second component including zinc (Zn), a leadedor a lead-free glass frit, and a resin binder that is dispersed in anorganic medium.

The second component is 2 to 5 wt % based on the total wt % of thecomposition. In addition, the second component can include at least oneselected from a group consisting of silicon (Si), aluminum (Al), copper(Cu), manganese (Mn), bismuth (Bi), phosphorous (P), boron (B), barium(Ba), and palladium (Pd).

The first component may be 80 to 85 wt % based on the total wt % of thecomposition, but it is not necessarily limited thereto. The phase of thefirst component or the second component is not particularly limited andmay be a liquid phase or a solid phase. Preferably, the phase of thefirst component or the second component may be a solid powder.

The glass frit may or may not include lead. If it includes lead, thelead content may be 1 to 3 wt % based on the total wt % of thecomposition.

The resin binder is a polymer plastic compound. Any material capable ofserving as a binder for binding with metal materials may be used.

The first component of 50 to 85 wt %, the second component of 2 to 5 wt%, the glass frit of 2 to 10 wt %, and the resin binder of 1 to 5 wt %based on the total wt % of the composition of the electrode paste forthe solar cell according to the one exemplary embodiment may bedispersed in an organic medium of 10 to 30 wt %. According to exemplaryembodiments, when the weight of the metal in the materials of the pasteis the total weight, silver (Ag) is 50 to 85 wt %, zinc (Zn) is 0.2 to 5wt %, and lead (Pb) is 0 to 2 wt %.

The high efficiency solar cell electrode according to one exemplaryembodiment can be formed by printing the electrode paste having theforegoing composition materials and composition ratio on a lightreceiving surface of the solar cell. The paste is then thermally fired

The electrode paste may be used for existing crystalline silicon solarcells and, in particular, both single crystal and polycrystallinesilicon solar cells. Further the electrode paste may be applied to asolar cell with high photovoltaic efficiency including an emitter layerhaving high sheet resistance. In order words, the paste can be used formanufacturing an electrode of a High Rs solar cell. Such solar cells mayhave an emitter layer with a high sheet resistance of 60 to 120Ω/square.

In crystalline silicon solar cells, the emitter layer formed on asubstrate forms a pn junction using the substrate as a base and, becausethe High Rs cell has the sheet resistance of 60 to 120 Ω/square, whichis higher than the emitter layer of existing crystalline solar cellswith a the sheet resistance of 40 to 50 Ω/square, it has excellentphotovoltaic efficiency. In other words, High Rs cells have a smallerdead layer (an area which hinders the generated electrons from forming acurrent due to an extra semiconductor impurity concentration) which isgenerated at the surface of the emitter layer adjacent the front surfaceof the substrate of the solar cell, thereby improving the efficiency ofthe solar cell.

The depth of the emitter layer of the High Rs cell may be 100 nm to 500nm and the semiconductor impurity concentration of the emitter layer maybe 1×10¹⁶ to 1×10²¹ atom/cm³. In the manufacturing process of solarcells according to the related art, the depth of the emitter is formedas a shallow ultra thin type layer as described above and thesemiconductor impurity is doped with a low doping concentration, makingit possible to manufacture the High Rs cell.

The depth of the emitter layer of general solar cells is 60 nm or more.The depth of the emitter layer of a High Rs cell is 100 nm to 500 nm.When the electrode is formed in a High Rs cell, the electrode passesthrough the shallow depth of the emitter layer and may then contact thebase electrode, such that a short circuit may more easily occur. Inorder to commercially use High Rs cells including a shallow depthemitter layer, a need exists for an electrode paste capable ofpreventing the short circuit state caused by contact between theelectrode and the silicon substrate, due to the excess firing.

Therefore, when using the electrode paste, the fire through is slowlyperformed during the thermoplastic process after application. In thisway, the electrode can delicately contact the shallow emitter layer,making it possible to manufacture the High Rs cell so that the qualityof the cell is maintained.

A solar cell according to another exemplary embodiment may comprise afirst conductive type semiconductor substrate; a second conductive typesemiconductor layer, formed on a front surface of the first conductivetype semiconductor substrate, having a conductive type opposite to thefirst conductive type; an antireflective layer that formed on the secondconductive type semiconductor layer; a front electrode that penetratesthrough the antireflective layerand is connected to the secondconductive type semiconductor layer; and a rear electrode that is formedon a rear surface of the first conductive type semiconductor substrate.

The depth of the second conductive type semiconductor layer is 100 nm to500 nm. In addition, the front electrode may include a first componentincluding silver (Ag) and a second component including zinc (Zn). Thesecond electrode may include at least one selected from a groupconsisting of silicon (Si), aluminum (Al), copper (Cu), manganese (Mn),bismuth (Bi), phosphorous (P), boron (B), barium (Ba), palladium (Pd),etc. as the second component. The front electrode may include a leadcomponent but it preferably does not include lead.

Further, in the front electrode the first component may be 75 to 95 wt%, the second component may be 2.5 to 8 wt %, and the glass frit may be2.5 to 17 wt %, based on the total wt % of the composition. Moreover,the front electrode may include silver (Ag) of 50 to 85 wt %, zinc of0.2 to 5 wt %, and lead (Pb) of 0 to 2 wt % based on the total wt % ofthe metal of the front electrode.

In the solar cells according to exemplary embodiments described herein,the first conductive type is a p type. The second conductive type may ben type, or vice versa. The semiconductor substrate may be a singlecrystal silicon substrate or a polycrystalline silicon substrate. Thesecond conductive type semiconductor layer uses the semiconductorsubstrate as a base and it serves as an emitter that forms a pn junctionat an interface with the substrate.

The second conductive type semiconductor impurity concentration of thesecond conductive type semiconductor layer may be 1×10¹⁶ to 1×10²¹atom/cm³. The second conductive type semiconductor impurityconcentration of the second conductive type semiconductor layer may havea concentration gradient from a high concentration to a lowconcentration within a range of 1×10¹⁶ to 1×10²¹ atom/cm³ going from thesurface to the pn junction interface. Because the second conductive typesemiconductor layer is as thin as 100 nm to 500 nm and has theconcentration gradient of the semiconductor impurity within the shallowemitter layer, the high concentration impurity doping area of thesurface of the emitter layer is relatively narrow compared to existingemitter layers, such that the dead layer is minimized to prevent therecombination of carriers.

Further, the sheet resistance of the second conductive typesemiconductor layer is 60 to 120 Ω/square. Thus, solar cells accordingto the exemplary embodiments may be High Rs cells. The second conductivetype semiconductor layer and the antireflective layer in the solar cellmay be formed in two layers or more. The rear electrode may be a metalother than zinc (Zn) and it may include aluminum (Al) or silver (Ag),but it is not limited thereto.

A solar cell according to exemplary embodiments may include a dielectriclayer formed on the rear surface of the first conductive typesemiconductor substrate. The rear electrode then penetrates through thedielectric layer and contacts the rear surface of the first conductivetype semiconductor substrate.

Further, according to exemplary embodiments, the rear electrode may beformed throughout the rear surface of the semiconductor substrate butmay be a back contact rear electrode formed by being partiallypatterned. In the case of the back contact rear electrode, the rear partof the semiconductor substrate in which the rear electrode does notexist is formed with the dielectric layer, making it possible to preventdamage to the solar cell.

In a solar cell according to exemplary embodiments, an interface betweenthe rear surface of the first conductive substrate and the rearelectrode may be formed with a back surface field (BSF). The backsurface field (BSF) may be formed at the interface between the rearelectrode and the semiconductor substrate regardless of the mountingform of the rear electrode, making it possible to have the rear fieldeffect of the solar cell in order to prevent the recombination of pairsof separated electrons and holes, thereby reducing the leakage current,so as to have good ohmic contact.

Still further, there is provided a fabrication method of a solar cellaccording to the exemplary embodiments, comprising: forming a secondconductive type semiconductor layer having a depth of 100 nm to 500 nmby doping a second conductive type semiconductor impurity opposite to afirst conductive type on a front part of a first conductive typesemiconductor substrate; forming an antireflective layeron the secondconductive type semiconductor layer; printing a front electrode paste onthe antireflection film; printing a rear electrode paste on a rearsurface of the first conductive type semiconductor substrate;heat-treating the front electrode paste to contact to the secondconductive type semiconductor layer by penetrating through theantireflection film; and heat-treating the rear electrode paste. Theheat treatment of the front electrode paste and the heat treatment ofthe rear electrode paste may be performed at the same time or atdifferent times.

The front electrode paste includes a first component including silver(Ag) or a metal alloy containing silver, a second component includingzinc (Zn), a leaded or lead-free glass frit, and a resin binder that isdispersed in an organic medium. According to one exemplary embodiment,the second component may further include at least one selected from agroup consisting of silicon (Si), aluminum (Al), copper (Cu), manganese(Mn), bismuth (Bi), phosphorous (P), boron (B), barium (Ba), andpalladium (Pd).

The rear electrode paste is not necessarily limited and may use a knownaluminum paste, aluminum silver paste, or the like. The rear electrodepaste may or may not include zinc (Zn). However, the process forms thefront electrode using an electrode paste according to exemplaryembodiment as described herein.

In the fabrication method the sheet resistance of the second conductivetype semiconductor layer formed on the semiconductor substrate may beformed in high resistance unlike the sheet resistance of the emitterlayer of existing solar cells, such that it can be manufactured as aHigh Rs cell.

The electrode paste can be applied to a general silicon solar cell aswell as High Rs cells and, in particular, when using the existingelectrode paste at the time of forming the front electrode of a High Rscell, the problems associated with short circuit caused during thefiring process are reduced.

The second conductive type semiconductor layer of a High Rs cell hashigh sheet resistance, wherein the sheet resistance is within a range of60 to 120 Ω/square. This is higher than the sheet resistance of theemitter of existing solar cells, which is 40 to 50 Ω/square. Further,the second conductive type semiconductor impurity concentration of thesecond conductive type semiconductor layer of the High Rs cell may be1×10¹⁶ to 1×10²¹ atom/cm³. Still further, in the fabrication method ofthe solar cell, the antireflective layermay be formed in two layers ormore.

The manufacturing method may further comprise forming a dielectric layeron the upper part of the second conductive type semiconductor layer orthe lower part of the first conductive type semiconductor substrate. Theprocess may be accomplished prior to the forming the rear electrode.Preferably, it may be formed prior to the forming the antireflectionfilm.

The dielectric layer may be silicon oxide (SiOx) or silicon nitride(SiNx), but it is preferably silicon dioxide (SiO₂). The dielectriclayer is formed by any one of atmospheric pressure chemical vapordeposition (APCVD), low pressure chemical vapor deposition(LPCVD),plasma enhanced chemical vapor deposition (PECVD), sputteringdeposition, electron beam deposition, and a spin-on process. However,the spin-on process is preferred.

The fabrication method may further include forming at least one openingin the dielectric layer after forming the dielectric layer, such thatthe rear electrode can contact the rear surface of the first conductivetype semiconductor substrate through the opening part. In other words,when forming the dielectric layer on the lower part of the firstconductive type semiconductor substrate, the process of forming the rearelectrode through the opening by patterning the opening should beaccomplished. The opening may be formed by patterning for example, aphotolithography method, an optical scribing method, a mechanicalscribing method, a plasma etching method, a wet etching method, a dryetching method, a lift-off method, and a wire mask method.

The fabrication method may also involve applying the rear electrodepaste on the dielectric layer after forming the dielectric layer andsimultaneously irradiating the dielectric layer and the rear electrodepaste layer using a laser, such that the rear electrode contacts therear surface of the first conductive type semiconductor substrate. Theirradiating time and strength of the laser are not particularly limitedbut must be sufficient if both the dielectric layer and the rearelectrode paste layer are fired by high heat and the rear electrodepenetrates through the dielectric layer to contact to the rear surfaceof the first conductive type semiconductor substrate.

The thermal firing temperature and time of the printed paste are notparticularly limited but, preferably, the thermal firingtemperature(heating temperature) may be 800° C. to 950° C. The thermalfiring time(heating time) may be 1 to 2 seconds.

The fabrication method may further comprise texturing the substrate whenpreparing the semiconductor substrate, wherein the texturing may be anyone of a wet chemical etching method, a dry chemical etching method, anelectrochemical etching method, or a mechanical etching method. But thefabrication method is not necessarily limited thereto.

The electrode paste has more excellent contact characteristics than theexisting electrode paste and controls the fire through in process offorming the electrode. In particular, an electrode paste is providedwhich can be applied to the High Rs cell including the emitter layerhaving a higher sheet resistance than existing silicon solar cells. Thepaste has excellent contact ability for the ultra-thin emitter layerdoped at the predetermined impurity doping concentration of the High Rscell.

Also, an emitter layer having high sheet resistance to improve thephotovoltaic efficiency of the entire solar cell can be provided, andthe development and industrial application of the electrode paste usingthe existing process in manufacturing the high efficiency solar cell canbe facilitated. Therefore, a high efficiency solar cell with improvedperformance at a lower cost or at least approximately equal to theproduction cost of existing solar cells can be provided.

Hereinafter, preferred embodiments will be described in detail withreference to the accompanying drawings. FIGS. 1 to 5 are cross-sectionalviews sequentially showing the fabrication method of a solar cellaccording to exemplary embodiments.

First, an n type emitter layer 2 is formed on the surface of a substrateby doping an n type semiconductor impurity on the p type semiconductorsubstrate 1.

Although not shown in FIG. 1, the n type semiconductor impurity is dopedon a front surface and a rear surface of the substrate as well as bothsides thereof. Thereafter, the doping layer on the side is removedthrough an edge isolation process and in a process of forming a rearelectrode, since the n type semiconductor impurity layer is compensated,FIG. 1 shows only the n type doping layer(n type emitter) 2.

The n type emitter layer 2 forms a pn junction at the interface with thep type semiconductor substrate 1. The n type emitter layer may be formedusing a deposition process and a known semiconductor impurity. The solarcell according to the exemplary embodiments may be a High Rs cell havingsheet resistance in the range of 60 to 100 Ω/square, wherein the n typeemitter layer 2 of the High Rs cell can control the thermal diffusionprocess of the n type semiconductor impurity so that the n typesemiconductor impurity concentration is 1×10¹⁶ to 1×10²¹ atom/cm³ andthe depth thereof is 100 nm to 500 nm.

Although not shown in FIG. 1, prior to the n type emitter depositionprocess, the surface of the p type semiconductor substrate 1 may besubjected to a texturing process. The texturing process may involve awet chemical etching method, a dry chemical etching method, anelectrochemical etching method, a mechanical etching method, or thelike. Because the substrate may be is an ultra-thin type substrate, thatis, the thickness is very small, the texturing process is more likely toinvolve a chemical etching method rather than a mechanical etchingmethod. In so doing, causing damage or defect to the substrate isminimized. Thereafter, the substrate is subjected to a cleaning processto remove an organic and/or inorganic material.

In the next process, as illustrated in FIG. 2, an antireflective layer3is formed on the n type emitter layer 2 which, in turn, was formed onthe front surface of the substrate 1. FIG. 2 shows that theantireflective layer3 is formed in a single layer. However, the processbut is not limited thereto. Therefore, the antireflective layer 3 maycomprise a plurality of layers. The material forming the antireflectivelayer is not particularly limited, but for to materials which canprevent light leakage out of the solar cell. Preferably, a silicon oxidefilm, a silicon nitride film, or a film comprising a mixture of both maybe used. The material used for the antireflective layer may,specifically, use a dielectric material, for example, a single layer ofSiNx, two layers of SiNx/SiON or SiNx/SiOx, or three layers ofSiOx/SiNx/SiOx. However, once again, the material is not limitedthereto. The antireflective layer 3 minimizes the reflectance of lightfrom the solar cell and it also serves as a passivation layer.

Next, a front electrode 4 is formed on the antireflective layer 3 bypatterning, as shown in FIG. 3. More specifically, the front electrode 4is formed by patterning the electrode paste on a predetermined area ofthe antireflection film 3, thermally firing the paste, causing it topassthrough the antireflection film, and making contact with the n typeemitter layer 2.

The electrode paste has a first component including silver (Ag), or ametal alloy containing Ag, a second component including zinc (Zn) and atleast one other component selected from the group consisting of silicon(Si), aluminum (Al), copper (Cu), manganese (Mn), bismuth (Bi),phosphorous (P), boron (B), barium (Ba), palladium (Pd), a leaded orlead-free glass frit, and a resin binder that is dispersed in an organicmedium. It is preferable that the second component is 2 to 5 wt % basedon the total wt % of the composition. Further, the first component is 80to 85 wt % based on the total wt % of the composition. Still further,the glass frit may or may not include lead, and if it does include lead,the content of lead may be 1 to 3 wt % based on the total wt % of thecomposition. When the weight of the metal in the electrode pastematerial makes up the total weight, silver (Ag) may be 50 to 85 wt %,zinc (Zn) may be 0.2 to 5 wt %, and lead (Pb) may be 0 to 2 wt %.

Table 1 below illustrates the wt % ratio of the metal for the electrodepaste. In Example 1, the electrode paste is lead (Pb)-free, asindicated. In example 2, the electrode paste contains lead.

TABLE 1 Wt % Ag Pb Zn Al Cu Mn Bi P Si B Ti Comparative 80 3.3 0.01 0.07<0.01 <0.01 0.01 0.04 0.2 0.01 <0.01 Example Example 1 80 — 0.005 0.010.005 0.09 3.5 0.01 0.13 0.025 — Example 2 85 0.31 0.2 0.095 0.005 0.0050.005 0.035 0.3 0.025 — Comparative Example: Existing front electrodepaste - leaded (Pb)

The weight ratios of the metal components for Examples 1 and 2, asindicated in Table 1, are exemplary, and not limiting. Preferably, theratio of zinc (Zn) in the paste is mixed in the range of 0.2 to 5 wt %as illustrated.

In the paste according to the above Table 1, the lead content in themetal largely decreases, while the zinc content increases when onecompares Example 2 with the comparative example. When Example 1 iscompared with the comparative example, bismuth (Bi) increases instead ofzinc. The content of bismuth increases so that the paste characteristicscan be better achieved.

FIG. 4 illustrates a rear electrode 5 being formed on the rear surfaceof the p type semiconductor substrate 1. Like the front electrode 4, therear electrode 5 is formed using the known electrode forming process.the rear electrode 5 may be formed at the same time or at a differenttime than the front electrode 4. Accordingly, an electrode paste isapplied and then thermally fired to complete the rear electrode 5.

The thermal firing temperature of the front electrode and the rearelectrode is 800° C. to 950° C. and the thermal firing time is 1 to 2seconds. Again, this range of temperatures is exemplary. The rearelectrode 5 may be formed by applying an aluminum (Al) paste or an alloypaste of aluminum silver (AlAg), or the like, and then thermally firingthe paste.

As shown in FIG. 5, the solar cell, according to exemplary embodiments,is subject to a thermoplastic process such that a back surface field(BSF) 6 is formed on the rear surface of the p type silicon substrate 1as a high concentration doping layer of the p type semiconductorimpurity. The back surface field (BSF) 6, which is doped with a p typeimpurity at a high concentration, serves to induce electron and holepairs that are separated by received light and then further separated toprevent the electron hole pairs from being re-combined. This makes itpossible to increase the efficiency of the solar cell.

FIG. 5 is a cross-sectional view of a solar cell having the rearelectrode S deposited over the rear surface of the semiconductorsubstrate 1. In contrast, FIG. 6 illustrates a back contact solar cellwhere the rear electrode 5 partially contacts the semiconductorsubstrate 1. FIG. 6 also illustrates that using the electrode paste mayinclude a dielectric layer 7 which serves as a passivation layer betweenthe n type emitter layer 2 and the antireflective layer3. A dielectriclayer 7 may also be formed on the rear surface of the semiconductorsubstrate 1 and then, the rear electrode 5 may be formed thereon.

After the dielectric layer 7 is formed, then openings are formed so thatthe rear electrode 5 partially contacts the substrate 1. The rearelectrode 5 partially contacts the substrate 1 through these openingswhich are in the back surface field (BSF) 6, as illustrated. Thedielectric layers 7 may be silicon oxide (SiOx) or silicon nitride(SiNx), but silicon dioxide (SiO₂) is preferable.

In the manufacturing method, the dielectric layer 7 may be formed by anyone of atmospheric pressure chemical vapor deposition (APCVD), lowpressure chemical vapor deposition (LPCVD), plasma enhanced chemicalvapor deposition (PECVD), sputtering deposition, electron beamdeposition, a spin-on process, or the like. However, a spin-on processis preferable. The openings 6 for the rear electrodes are formed by aphotolithography method, an optical scribing method, a mechanicalscribing method, a plasma etching method, a wet etching method, a dryetching method, a lift-off method, a wire mask method, or the like.

FIG. 7 shows a flow chart of a manufacturing method, according toexemplary embodiments for a solar cell as described above. Thefabrication method involves forming an unevenness structure (i.e.,texture) on the substrate 1 by employing a texturing process that willminimize possible damage to the substrate surface (S10). The texturedsurface increases the light receiving area to increase, in turn, lighttrapping.

Thereafter, a cleaning process for removing organic and/or inorganicmaterial is accomplished (S20). Then, in order to manufacture an inverseconductive n type emitter layer, a pn junction structure is formed bythermally diffusing n type semiconductor impurities of group-V elementssuch as phosphorus (P), arsenic (As), antimony (Sb), and the like, ontothe p type substrate (S30). The process of thermal diffusion generallyforms an emitter layer of 500 nm or more but, in the present invention,it is important to maintain the thermal diffusion depth of the pnjunction to a depth of 100 to 500 nm.

Thereafter, a silicon nitride film, which serves as an antireflectivecoating on the n type emitter layer, is formed. To achieve this, aprocess such as plasma enhanced chemical vapor deposition (PECVD) may beemployed (S40).

A silver (Ag) electrode is then formed on the antireflective coating bya screen printing process, while an aluminum (Al) rear electrode is alsoformed using a screen printing or similar process (S50). Morespecifically, an electrode paste according to exemplary embodiments isapplied to the front part of the light receiving area using a screenprinting method. The sequence of forming the rear electrode and thefront electrode is not limited to forming either electrode before theother.

After the screen printing process, a drying process (S60) and an edgeisolation process are performed (S70) and the high temperaturesimultaneous firing proceeds in order to form the back surface field(BSF) on the rear surface as explained above (S80). The thermal firingprocess increases the contact between the front electrode and the frontsurface. The thermal firing process for the front part and the rear partdoes not have to be a simultaneous firing process and they may each befired separately.

Again, the method illustrated in FIG. 7 is exemplary. The sequence ofthe method is only one embodiment and is not limited thereto.

FIG. 8 is a graph of comparing the doping concentrations ofsemiconductor impurity according to the sputter depths of emitter layersin a High Rs cell and an existing well-known solar cell. The graph ofFIG. 8 shows results based on data relating to an n type semiconductorimpurity diffused into the wafer according to the thickness of the ntype emitter layer, in particular, the concentration of phosphorus (P),in the process for forming the pn junction among the manufacturingprocesses of the solar cell manufactured using a 156 mm single crystalwafer.

In an existing solar cell, the data for which is indicated by -♦-, itcan be appreciated that the n type impurity concentration of the emitterlayer is formed at a high concentration of 1.0×10²¹ atoms/cm³ at thesurface and then is diluted to the concentration of 1.0×10¹⁶ atoms/cm³.In particular, it can be appreciated that when 1.0×10¹⁷ atoms/cm³ isconsidered as a reference, the depth of the emitter layer is 600 nm ormore.

In comparison, the High Rs cell, according to exemplary embodiments ofthe present invention, the data for which is are indicated by -▴-, itcan be appreciated that when 1.0×10¹⁷ atoms/cm³ is considered as areference, it has an emitter layer having a very thin depth where thesputter depth is about 350 nm. Further, the High Rs cell has an emitterlayer with high sheet resistance, which is 60 Ω/square or more and ascan be appreciated from FIG. 8, it has the depth of the n type impuritydoping section that is even thinner than the existing solar cell, makingit possible to implement an ultra-thin solar cell and improve thephotovoltaic efficiency.

In addition, the high concentration doping section at the surface of theemitter layer is considered a dead layer and, as shown in FIG. 8, the ntype impurity doping concentration is a high concentration 1.0×10²¹atoms/cm³ to 1.0×10²² atoms/cm³. Because the corresponding sectionhinders the generated electrons from forming a current due to the extran type impurity concentration, the dead layer section should be short toimprove efficiency. As can be appreciated from FIG. 8, the High Rs cellhas the relatively short dead layer section compared to existing solarcells, such that the electron focusing probability increases, making itpossible to improve the efficiency.

The paste used for forming the front electrode which contacts the verythinly formed emitter layer of the High Rs cell was described above withreference to Table 1. Using a paste having components as shown in Table1 are suitable and of sufficient efficiency for High Rs cells.

Comparing the efficiency between solar cells manufactured by applyingthe paste according to exemplary embodiments and existing solar cellsmanufactured using existing electrode paste is as follows in Tables 3and 4.

TABLE 2 Jsc Voc (mA/cm²) (mV/cm²) FF (%) Efficiency (%) ConventionalCell 32.2 611 77.3 15.31 High Rs Cell-paste A 32.8 615 77.4 15.62

TABLE 3 Jsc Voc (mA/cm²) (mV/cm²) FF (%) Efficiency (%) ConventionalCell 33.85 622 79.5 16.75 High Rs Cell-paste A 34.6 625 79 16.96

With reference to Table 2, both the existing (i.e., conventional) solarcell and the High Rs cell use a 156 mm polycrystalline silicon wafer. InFIG. 3, a 156 mm single crystal silicon wafer is used. Unlike theexisting solar cell, the High Rs cell is manufactured by forming thefront electrode using paste A according to exemplary embodiments of thepresent invention, as described above. In tables 2 and 3, paste Acomprises the metal composition of Example 2 in Table 1.

With continued reference to Tables 2 and 3, the difference inphotovoltaic efficiency, as shown, is caused by the difference betweenthe single crystal silicon and polycrystalline silicon wafer. Generally,the efficiency of the single crystal silicon solar cell is greater.However, the efficiency of the High Rs cell, which uses paste Aaccording to exemplary embodiments of the present invention, is greaterthan the efficiency of the existing solar cell regardless whether singlecrystal silicon or polycrystalline silicon is employed.

When a High Rs cell uses existing (i.e., conventional) paste for thefront electrode, the electrode passes through the very thinly formedemitter layer to reach and contact with the base substrate. Theelectrode and the substrate are thus short-circuited, and the solar cellcannot be manufactured. Therefore, it is realized that in themanufacturing process of a silicon solar cell, and in particular, a highefficiency solar cell such as the High Rs cell described above, thepaste more smoothly controls the speed at which the electrode is formed.

Those skilled in the art will appreciate that various modifications andvariations can be made in the present invention without departing fromthe spirit or scope of the inventions. Also, the substances of eachconstituent explained in the specification can be easily selected andprocessed by those skilled in the art from the well-known varioussubstances. Also, those skilled in the art can remove a part of theconstituents as described in the specification without deterioration ofperformance or can add constituents for improving the performance.Furthermore, those skilled in the art can change the order to methodicsteps explained in the specification according to environments ofprocesses or equipment. Thus, it is intended that the present inventioncovers the modifications and variations of this invention provided theycome within the scope of the appended claims and their equivalents.

1. A solar cell comprising: a semiconductor substrate comprising a firstconductive type material; a semiconductor layer comprising a secondconductive type material formed on a front surface of the semiconductorsubstrate wherein the first conductive type material is different thanthe second conductive type material; an antireflective layer formed onthe semiconductor layer; a front electrode passing through theantireflective layer and contacting the semiconductor layer; and a rearelectrode formed on a rear surface of the semiconductor substrate,wherein the depth of the semiconductor layer is 100 nm to 500 nm andwherein the front electrode comprises a first component including silver(Ag) and a second component including zinc (Zn).
 2. The solar cellaccording to claim 1, wherein the first component is 75 to 95 wt % andthe second component is 2.5 to 8 wt % based on the total wt % of thefront electrode composition.
 3. The solar cell according to claim 1,wherein the front electrode further includes lead (Pb), and whereinsilver (Ag) is 50 to 85 wt %, zinc (Zn) is 0.2 to 5 wt %, and lead (Pb)is equal to or less than 2 wt %. based on the total metal wt % of thefront electrode.
 4. The solar cell according to claim 1, wherein thefirst conductive type material is p type and the second conductive typematerial is n type.
 5. The solar cell according to claim 1, wherein thesheet resistance of the semiconductor layer is 60 to 120 Ω/square. 6.The solar cell according to claim 1, wherein the impurity concentrationof the second conductive type material of the semiconductor layer is1×10¹⁶ to 1×10²¹ atom/cm³.
 7. The solar cell according to claim 1,wherein the impurity concentration of the second conductive typematerial of the semiconductor layer has a concentration gradient in therange of 1×10¹⁶ to 1×10²¹ atom/cm³ from a front surface of theconductive layer to a pn junction interface between the conductive layerand the conductive substrate.
 8. The solar cell according to claim 1,wherein the antireflective layer comprises a plurality of layers.
 9. Thesolar cell according to claim 1, wherein the composition of the frontelectrode is different than the composition of the rear electrode. 10.The solar cell according to claim 9, wherein the composition of the rearelectrode includes aluminum (Al) or silver (Ag).
 11. The solar cellaccording to claim 1, wherein the composition of the rear electrodeexcludes zinc (Zn).
 12. The solar cell according to claim 1, wherein adielectric layer is formed between the front electrode and theantireflection layer, and wherein the rear electrode passes through thedielectric layer and makes contact with the rear surface of thesemiconductor substrate.
 13. The solar cell according to claim 1,wherein a back surface field (BSF) is formed between the semiconductorsubstrate and the rear electrode.
 14. A solar cell fabrication methodcomprising: forming a semiconductor layer having a depth of 100 nm to500 nm by doping a front surface of a semiconductor substrate using asecond conductive type material impurity, wherein the semiconductorsubstrate comprises a first conductive type material and the firstconductive type material is different from the second conductive typematerial; forming an antireflective layer on the semiconductor layer;printing a front electrode paste on the antireflective layer; printing arear electrode paste on a rear surface of the semiconductor substrate;heat-treating the front electrode paste thereby causing a resultingfront electrode to penetrate through the antireflective layer and makecontact with the semiconductor layer, wherein the composition of thefront electrode paste limits the penetration of the front electrode tothe semiconductor layer; and heat-treating the rear electrode pastethereby resulting in the formation of a rear electrode.
 15. Thefabrication method according to claim 14, wherein heat-treating thefront electrode paste and heat-treating the rear electrode paste isperformed at the same time.
 16. The fabrication method of claim 14,wherein the composition of the front electrode paste is 2-5 wt % basedon the total wt % of the front electrode paste.
 17. The fabricationmethod according to claim 14, wherein the front electrode paste includessilver (Ag) or a metal alloy containing silver (Ag).
 18. The fabricationmethod according to claim 17, wherein the front electrode paste furtherincludes glass frit which comprises zinc (Zn) and at least one componentselected from a group consisting of silicon (Si), aluminum (Al), copper(Cu), manganese (Mn), bismuth (Bi), phosphorous (P), boron (B), barium(Ba), and palladium (Pd).
 19. The fabrication method according to claim14, wherein the sheet resistance of the second conductive typesemiconductor layer is 60 to 120 Ω/square.
 20. The fabrication methodaccording to claim 14, wherein the second conductive type semiconductorimpurity concentration is 1×10¹⁶ to 1×10²′ atom/cm³.
 21. The fabricationmethod according to claim 14, wherein the antireflective layer comprisesa plurality of layers.
 22. The fabrication method according to claim 14further comprising: forming a dielectric layer directly on the rearsurface of the semiconductor substrate.
 23. The fabrication methodaccording to claim 22 further comprising: forming at least one openingin the dielectric layer, wherein the rear electrode passes through theat least one opening and makes contact with the rear surface of thesemiconductor substrate.
 24. The fabrication method according to claim22, wherein the rear electrode paste is applied on the dielectric layerand the dielectric layer and the rear electrode paste layer aresimultaneously irradiated by laser thereby causing the rear electrode tomake contact with the rear surface of the semiconductor substrate. 25.The fabrication method according to claim 14, wherein the heat-treatingtemperature is 800 to 950° C. and the heat-treating time is 1 to 2seconds.